Vinylidene fluoride resin is excellent in chemical resistance, heat resistance and mechanical strength and, therefore, has been studied with respect to application thereof to porous membranes for separation. In the case of use for water treatment, particular for production of potable water or sewage treatment, a hollow fiber-form porous membrane is frequently used because it can easily provide a large membrane area per unit volume of filtration apparatus.
For the above purpose, a porous membrane is required to show a tensile strength and an elongation at break which are large to some extent so as not to cause fiber severance not only during the filtration operation as a matter of course but also during physical washing, such as back washing or air scrubbing, which is performed to remove clogging of membrane with time.
As the physical washing does not achieve sufficient washing effect against clogging with organic matter, back washing with water containing sodium hypochlorite or ozone or periodical washing with chemicals is also performed. Further, in some case, a filtration operation is performed by adding sodium hypochlorite or ozone to raw water (supply water). Accordingly, a porous membrane is required to have a high chemical resistance so as not to lower its mechanical strength (tensile strength, elongation at break) due to such chemicals for a long period.
As for improvement in mechanical strength and chemical resistance, Patent document 1 listed below contains descriptions to the effect that a porous membrane comprising vinylidene fluoride homopolymer becomes a fragile membrane and that a copolymer shows insufficient mechanical strength unless the content of vinylidene fluoride monomer is substantially lowered, but this results in inferior chemical resistance.
Patent document 2 below describes that a porous membrane having excellent resistance against oxidation with ozone is a membrane showing a maximum peak temperature (of melting point) of at least 160° C. and a higher maximum peak temperature leads to a better ozone resistance. The document also describes that a porous membrane comprising a PVDF resin having a weight-average molecular weight of less than 105 exhibits an extremely low tensile elongation at break.
From these patent documents, it is suggested that (1) a higher crystallinity results in a better chemical resistance and (2) a high-molecular weight polymer results in improved mechanical strength.
The research group of the present inventors has proposed a process of melt-extruding a high molecular weight vinylidene fluoride resin having an inherent viscosity of 1.3-15 dl/g (corresponding to Mw (weight-average molecular weight) of 3.8×105-51×105) together with a plasticizer and preferably also with a good solvent, cooling the extruded film with a chill roll at a temperature of at most 150° C. from one side and with air from the other side to form a film, and extracting the plasticizer (Patent document 3 below). However, a porous membrane of a sufficiently high crystallinity has not been attained because of a tendency that a higher molecular weight results in a lower crystallinity due to a lower mobility of molecular chain at the time of crystallization.
Accordingly, it is a present state that a porous membrane of vinylidene fluoride resin excellent in both mechanical strength and chemical resistance has not been obtained.
Patent document 1: JP-A 11-152366
Patent document 2: JP-A 2000-218267
Patent document 3: JP-A 7-173323